Cell for the electrolysis of alumina at low temperatures

Abstract

An electrolysis cell for the production of aluminum by the electrolysis of alumina dissolved in a molten halide electrolyte using a multimonopolar arrangement of substantially non-consumable anodes and cathodes with facing operative surfaces which are upright or at a slope and are spaced in a substantially parallel relationship, enabling operation at low and/or current density with an acceptable production per unit self-lowered area. The operative surface area of the anodes and the cathodes is high due to their upright sloping configuration. Operative surfaces of the anodes and possibly of the cathodes can be increased by making them porous, preferably with their articulated skeletal structure, e.g. with a porous active part on opposite faces of a central current feeder. There is an upward circulation of electrolyte by gas lift between the electrodes, such circulation being enhanced by electrolyte circulation guide means provides by members of electrically non-conducting material arranged outside the spacing between the anodes and the cathodes.

We claim: 
     
       1. An electrolysis cell having a top and a bottom, for the production of aluminium by the electrolysis of alumina dissolved in a molten salt electrolyte containing halides using substantially non-consumable anodes cooperating with a cathode arrangement in a multimonopolar arrangement of interleaved anodes and cathodes, said anodes and cathodes having sides and ends, said anodes and cathodes also having facing operative surfaces which are upright and in spaced substantially parallel relationship, wherein the spacing between the facing active anode and cathode surfaces is arranged for upward circulation of electrolyte by gas lift, and wherein spaces are provided outside the multipolar arrangement of anodes and cathodes for downward circulation of electrolyte and for replenishment of alumina in the electrolyte, comprising: members of electrically non-conducting material arranged adjacent the edges of the facing anodes, to form electrolyte circulation guide means.   
     
     
       2. An aluminium production cell according to claim 1, wherein the multimonopolar arrangement of anodes and cathodes has means for electrical connection to the anodes (1) at the top of the cell, and means for electrical connection to the cathodes (1) at the bottom of the cell. 
     
     
       3. An aluminium production cell according to claim 2, wherein the cathodes dip into a cathodic aluminium layer on the bottom of the cell, the cell bottom having means for providing electrical connection of the aluminium layer to an external current supply. 
     
     
       4. An aluminium production cell according to claim 2 or 3, wherein at least the operative surfaces of the anodes, the cathodes or the cathodes and the anodes are high surface area structures. 
     
     
       5. An aluminium production cell according to claim 4, wherein the anodes, the cathodes or the anodes and cathodes have a central current feeder and a porous active part (13) on opposite faces of said central current feeder. 
     
     
       6. The aluminium production cell of claim 4, wherein said high surface area structures are porous. 
     
     
       7. The aluminium production cell of claim 6, wherein said high surface area structures are reticulated skeletal structures. 
     
     
       8. An aluminium production cell according to claim 1, wherein said spaces are arranged at the sides or ends of the multimonopolar arrangement of anodes and cathodes. 
     
     
       9. An aluminium production cell according to claim 1, wherein several multimonopolar arrangements of anodes and cathodes are arranged side-by-side with the spaces therebetween. 
     
     
       10. An aluminium production cell according to claim 1, comprising means for feeding alumina into the spaces to replenish the electrolyte with alumina. 
     
     
       11. An aluminium production cell according to claim 1, wherein the electrolyte circulation guide means comprise generally vertical bars of electrically non-conductive material adjacent the edges of the facing anodes and cathodes. 
     
     
       12. An aluminium production cell according to claim 1, wherein the electrolyte circulation guide means comprise at least one plate of electrically non-conductive material generally perpendicular to and on either side of the multimonopolar arrangement of anodes and cathodes. 
     
     
       13. An aluminium production cell according to claim 1, wherein the active anode surface have an area which is greater than the area of the facing active cathode surface. 
     
     
       14. An aluminium cell according to claim 1, wherein the electrolyte is a fluoride melt, or a mixed fluoride-chloride melt. 
     
     
       15. The cell of claim 14, wherein the electrolyte is a mixture of AlF 3  with at least one of NaF and LiF, comprising 42-63 weight % of AlF 3 , up to 48 weight % of NaF and up to 48 weight % of LiF. 
     
     
       16. The cell of claim 14, wherein the electrolyte is a mixed fluoride-chloride electrolyte comprising 90-70% by weight of one of more fluorides of sodium, potassium, lithium, calcium and aluminium with 10-30% by weight of one or more chlorides of sodium, potassium, lithium, calcium and aluminium. 
     
     
       17. The cell of claim 1 wherein each said anode and said cathode have a total facing active surface area and a horizontal projected area, wherein the total facing active surface areas of the anodes and cathodes are substantially equal and are each at least 1.5 times the horizontal projected area of the anodes and cathodes on the cell bottom. 
     
     
       18. The cell of claim 17 wherein the total facing active surface areas of the anodes and the cathodes are greater than or equal to 4 times the horizontal projected area of the anodes and cathodes on the cell bottom. 
     
     
       19. An electrolysis cell having a top and a bottom, for the production of aluminium by the electrolysis of alumina dissolved in a molten salt electrolyte containing halides using substantially non-consumable anodes cooperating with a cathode arrangement in a multimonopolar arrangement of interleaved anodes and cathodes, said anodes and cathodes having sides and ends, said anodes and cathodes also having facing operative surface which are upright and in spaced substantially parallel relationship wherein the spacing between the facing active anode and cathode surfaces is arranged for upward circulation of electrolyte by gas lift, and wherein spaces are provided outside the multimonopolar arrangement of anodes and cathodes for downward circulation of electrolyte and for replenishment of alumina in the electrolyte, said cell also comprising members arranged outside said spacing, to form electrolyte circulation guide means, wherein the electrolyte in said cell is maintained at an operating temperature in the range 680°-880° C. 
     
     
       20. The cell of claim 19 wherein current is supplied to the active anode surfaces at an anode current density below or at the threshold value for halide evolution. 
     
     
       21. The cell of claim 19, wherein the anode current density is from 0.1 to 0.4 A/cm 2  per unit area of the active anode surface area.